APU Generator switch to PWR

So apparently the engines still turn on without doing that, so what is the point of it ?

Edited March 31, 2020 by PinkyRng

Each engine also has its own generator. Once you get the engines running, and turn on their generators, you can turn off the APU generator and the APU.

....

Left engine starts with only Battery ON, Boost pumps ON, APU ON, Inverter STBY.

 

Right Engine doesn't start with only Battery ON, Boost pumps ON, APU ON, Inverter STBY.

 

Right Engine starts with only Battery ON, Boost pumps ON, APU ON, APU Gen ON, Inverter STBY.

 

Why is that?

 

....

It is just something related to the aircraft systems (Electric and power sources to relative buses, Fuel and Fuel Pumps...)

If for whatever reason you want to start up the right engine first with just Battery, Inverter and APU not providing Electrical power, you must turn on the Fuel Crossfeed with the boost pumps.

I believe the left engine has a dc powered boost pump which allows it to be started with just the battery bus (and instrument ac bus) energized.

 

 

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So apparently the engine turn of without doing that, so what is the point of it ?

 

It’s to power systems prior to being start, in an emergency, or for maintenance purposes. Also on the real jet with the generator off the cooling fan won’t turn on and you can burn up the APU hydraulic pump. Although that isn’t modeled in the sim.

 

I believe the left engine has a dc powered boost pump which allows it to be started with just the battery bus (and instrument ac bus) energized.

 

 

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No the DC powered pump is just for the APU.

I am guessing because he hadn't have bleed air on ?

That's interesting. As a noob I never tested that. Always get the APU GEN running asap so I can start CDU alignment. But I'd love to know why the right engine wouldn't fire up without APU generator.

Fuel supply. It's as Peaches says in post #6.

I agree there is a good open question about why the right engine wont start with left engine running, crossfeed on, but no apu generator running.

 

Does anyone have the electrical systems diagram? I have several copies of T.O. 1A-10A-1-1 which make reference to “Figure FO-5”, but I cant find that figure.

Then post #7 says that’s not correct.

 

The guy from post 7 has worked A-10's for 14 years so I'd go with what he said ;)

Edited March 29, 2020 by Snoopy

Great answer Snoopy - brightened my day up

 

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So, can he answer my question? :)

 

Why do one engine need apu gen ON while the other doesn't?

 

I haven’t tested in DCS but real world starting either engine has nothing to do with the APU generator being on (or off).

 

I’ll see if I can duplicate what you’re seeing.

I haven’t tested in DCS but real world starting either engine has nothing to do with the APU generator being on (or off).

 

I’ll see if I can duplicate what you’re seeing.

 

When going through the startup, I normally have the APU GEN on so the CDU/EGI are initializing. There have been times where I have forgotten to switch on both sets of main and wing boost pumps. With the boost pumps off, I typically don't catch that until I attempt to start engine 2. Engine 1 will start with the boost pumps off but not engine 2, it will just motor along in the start cycle. At that point while watching the gauges, wondering why the ITT hasn't shown ignition, that's when I'll have that "duh" moment and realize I forgot to turn the pumps on.

 

From what I recall from one of your past posts Snoopy, is that the DC pump in the left main tank will feed the APU and #1 engine, but not #2 unless the crossfeed is on. And IIRC the DC pump is automatic and activates if there is no pressure from the left main AC boost pump.

Edited March 31, 2020 by Diesel_Thunder

It is just something related to the aircraft systems (Electric and power sources to relative buses, Fuel and Fuel Pumps...)

If for whatever reason you want to start up the right engine first with just Battery, Inverter and APU not providing Electrical power, you must turn on the Fuel Crossfeed with the boost pumps.

 

For sake of personal knowledge

 

as stated in general on my answer, following is the specific answer

 

If for whatever reason on A10C we need to start the right eng first without the APU AC Generator ON (BUT with APU operating) we need to OPEN the Fuel Crossfeed Valve.

 

The Left Engine do not need the Fuel Crossfeed Valve opened and as the right Eng the APU Gen ON, for the following tec reasons (anyway the APU 5 min limit operation without APU GEN ON is still limiting).

 

This is due to Aircraft System logic and the availability of AC Fuel Boost Pumps (connected to AC Main Buses) and fuel pressurization.

 

With only Battery Power ON and Inverter ON just some basic and essential Buses are powered, so we have talking about Fuel Sys only a DC Fuel Pump pressurizing fuel that is located just in Left Main Tank.

 

So during Right Eng Start Up sequence we need to pressurize also the right Eng fuel line (possible with the Fuel Crossfeed Valve Open)

 

 

Normal Ops Engines Start Seq are with APU ON and also APU Gen ON (the only source for a fan cooling sys of APU hydraulic Pump) so to have AC Fuel Boost Pumps working on both Fuel Sys pressurizing respective fuel lines.

 

DCS is simulating that, I do not know if in real life is it possible to have a successful Right Eng Start with APU Gen OFF/Crossfeed OFF and just suction feed... but here we need some real A10C field test... :)

 

hope my explanation is clear.

Looking in our DCS A-10 flight manual, the DC boost pump is powered from the DC ESSEN bus ( page 658 ), which is powered by the battery when the battery power switch is on, or external power when the switch is off and external power is available and connected. APU GEN on or off does not affect this pump.

 

Here is a fuel system diagram that Snoopy posted in the past. If you look at the Left Main tank, you'll see the DC boost pump. Follow its output and you'll see that it goes to three places; the APU, left engine, and the crossfeed valve. Only when that crossfeed valve is open will fuel go to the right engine from the DC pump.

 

Here's a track that I saved where I tested this using the DC pump only. I started the APU but did not turn on the APU GEN. Left engine started normally. Right engine only motored when attempting to start with the crossfeed valve closed. Stopped the right engine, and then opened the crossfeed valve and attempted to start the right engine again. The engine started this time.

 

Sequence of events in the track:

 

• 1:00 - Left engine start, ignites normally
  
• 1:53 - Left engine start sequence finished
  
• 2:05 - Right engine start, engine only motors with no ignition
  
• 3:00 - Right engine stop (I waited for N1 and Fan speeds to drop to 0)
  
• 3:40 - Open crossfeed valve
  
• 3:45 - Right engine start, ignites normally
  
• 4:30 - Right engine start sequence finished
  

a-10c DC pump.trk

Edited June 18, 2020 by Diesel_Thunder

 

Here is a fuel system diagram that Snoopy posted in the past. If you look at the Right Main tank, you'll see the DC boost pump. Follow its output and you'll see that it goes to three places; the APU, left engine, and the crossfeed valve. Only when that crossfeed valve is open will fuel go to the right engine from the DC pump.

 

The DC fuel pump is located in the Left Main tank

 

 

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The DC fuel pump is located in the Left Main tank

 

 

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Thanks for catching that :thumbup:, I corrected it. Freudian slip on my part.

....Freudian slip on my part.

 

was obvious a slip, Just point it out to clear any confusion.

 

:thumbup:

@Diesel_thunder that diagram is the answer, PERFECT! And its from the same sequence as I was looking for that FO-5. Now I must find the rest of those figures. @Snoopy?

Edited March 30, 2020 by Puma
Typo

Thank you that is a good answer from post 7, shame they didn't simulate it. When I was teaching my friend how to turn on the A10 he asked me that question when he turned on the plane without fliping that switch.

Also I just saw my initial post had typos. What I meant was that the engine turns on without ever using it.

That's interesting. As a noob I never tested that. Always get the APU GEN running asap so I can start CDU alignment. But I'd love to know why the right engine wouldn't fire up without APU generator.

 

Originally Posted by =4c=Nikola

I do not understand the question, but I've tested some combinations and results are:

 

1. Left engine starts with only Battery ON, Boost pumps ON, APU ON, Inverter STBY.

 

2. Right Engine doesn't start with only Battery ON, Boost pumps ON, APU ON, Inverter STBY.

 

3. Right Engine starts with only Battery ON, Boost pumps ON, APU ON, APU Gen ON, Inverter STBY.

 

Why is that?

 

4. PS. Once started, both engines work without any elec. power (ENG Gens OFF, APU Gen OFF, Inverter OFF, Battery OFF)

 

5. PSS. APU works with just battery power ON, but it automatically shuts down when battery power is switched to OFF and restarts itself when you reapply battery power to ON

Just finishing gathering and compiling some information regarding the fuel pumps from manuals, diagrams and, and some of Snoopy's posts.

 

The DC pump automatically runs when the Left Main AC boost pump is not providing pressure. The DC pump is on the DC ESSEN bus which means it gets power and starts running as soon as the battery is turned on, or is connected to external power (DCS A-10C flight manual, page 658 ). The way the DC pump is plumbed into the system, it feeds the APU, left engine, and crossfeed valve. The only time the DC pump won't run is if the Left Main AC pump is running, the DC pump is damaged/broken, or you pulled the DC pump breaker on the breaker panel.

 

The main AC pumps (left/right main and wing pumps) are not connected to the inverter bus and cannot be run from that power source (DCS A-10C flight manual page 657). Only the APU generator, left and right engine generators, or ground power can supply power to these pumps. The inverter will run as long as the switch is in the STBY position and no AC power is available. Once a generator or ground power is online, the inverter shuts down and goes into standby mode.

 

From what I also understand, the DC pump is only intended to feed the APU so it can start, and get the AC pumps running for the main engines. I don't know how much fuel that DC pump can supply or if that pump alone can supply the engines running at full power. Though looking at the circuit breaker rating of 5 amps, I doubt that pump is large enough to feed the main engines any significant quantity of fuel. I bet the engine driven pumps are pulling fuel on their own once running if the DC pump is the only electric pump running. Snoopy, could you shed some light on this please?

 

So for the quoted questions above, I numbered them:

 

1 - Left engine will start in this case with just the DC pump. This question does not mention the position of the APU GEN switch, so I am assuming it is off. The inverter is still running at this point so the AC pumps are not running regardless that they are switched "on".

 

2. Right engine will not start if the DC pump is the only pump running AND the crossfeed valve closed. This question also does not mention the position of the APU GEN switch, so I am assuming it is off. The inverter is still running at this point so the AC pumps are not running regardless that they are switched "on".

 

3. In this case since the APU GEN is on, the AC boost pumps can come alive and supply fuel since they are also switched on, and the right engine will start normally.

 

4. This confirms to me that the engine driven pumps will supply fuel to themselves even without the electric pumps, at least in the sim.

 

5. The APU shuts down because not only did you turn off the DC pump that feeds it fuel, but you also turned off the APU controller.

 

Hopes this helps :)

Edited April 1, 2020 by Diesel_Thunder

It’s to power systems prior to being start, in an emergency, or for maintenance purposes. Also on the real jet with the generator off the cooling fan won’t turn on and you can burn up the APU hydraulic pump. Although that isn’t modeled in the sim.

 

 

 

No the DC powered pump is just for the APU.

 

 

 

Hey Snoopy, thanks for the response on the DC-powered boost pump. I understand that normal procedure is that the DC pump is used in the starting of the APU, which then is able to power the L/R AC Buses, which power the four AC boost pumps, which support main engine start.

 

 

I am asking about design philosophy, though, as the -1 (p. 1-19 of the version on the internet) states: "With the battery switch in PWR, the DC boost pump is energized when the APU switch is positioned to START, or either throttle is forward of OFF, and the left main boost pump is inoperative. The DC boost pump supplies fuel to the APU and the left engine."

 

 

From that, and looking at the engine start requirements and electrical bus design (FO-5 was helpful), it appears to me that the design philosophy of the DC boost pump was to allow left engine start without the APU, if ground air is available to power the ATS.

 

 

I look forward to your response, and we appreciate your keeping us armchair aficionados in line!

From what I also understand, the DC pump is only intended to feed the APU so it can start, and get the AC pumps running for the main engines. I don't know how much fuel that DC pump can supply or if that pump alone can supply the engines running at full power. Though looking at the circuit breaker rating of 5 amps, I doubt that pump is large enough to feed the main engines any significant quantity of fuel. I bet the engine driven pumps are pulling fuel on their own once running if the DC pump is the only electric pump running. Snoopy, could you shed some light on this please?

 

4. This confirms to me that the engine driven pumps will supply fuel to themselves even without the electric pumps, at least in the sim.

 

That is a correct assumption. The fuel pumps on the engines are strong enough to self feed once started. They are mechanically tied to the engine compressor through the gearbox so if the compressor goes faster, the pumps go faster, more fuel, etc. As far as being able to prime the lines enough for startup, I'm not 100% sure. I am reaching way back to Tech School without direct references handy. I believe you would risk cavitating the pump so you would need something to pressurize the feed before hand. Hence the meat of the current discussion.

 

Hey Snoopy, thanks for the response on the DC-powered boost pump. I understand that normal procedure is that the DC pump is used in the starting of the APU, which then is able to power the L/R AC Buses, which power the four AC boost pumps, which support main engine start.

 

 

I am asking about design philosophy, though, as the -1 (p. 1-19 of the version on the internet) states: "With the battery switch in PWR, the DC boost pump is energized when the APU switch is positioned to START, or either throttle is forward of OFF, and the left main boost pump is inoperative. The DC boost pump supplies fuel to the APU and the left engine."

 

 

From that, and looking at the engine start requirements and electrical bus design (FO-5 was helpful), it appears to me that the design philosophy of the DC boost pump was to allow left engine start without the APU, if ground air is available to power the ATS.

 

 

I look forward to your response, and we appreciate your keeping us armchair aficionados in line!

That seems like a reasonable conclusion. The APU has nothing to do with engine start other than air supply to the starters. Taking it further, with crossfeed on, you should also be able to start #2 in the same everything-off-except-battery, air connected, scenario just as Diesel described in post #16. My take away so far is the DC pump is simply a system primer/as well as a direct feed to the APU.

 

Fascinating. Everything past the engine pylon hookups is mud to me. I look forward to learning more.

 

Addendum: A recent oopsy on test cell reminded me that even though the engine can start up without boost pumps, it is more of a function of residual fuel in the lines the engine pump sucks out than able to take it from the tank itself. The motor will die at idle without pumps on :smilewink:

Edited April 25, 2020 by zinhawk
More brain space

Just want to clarify here - the TF34/older CF34 has no problem suction feeding from the fuel tanks. It'll happily run fine at all power settings. There is an operating limitation though of 10 - 50 hours depending on airframe installation. This is because it is hard on the engine driven fuel pump and there is risk of cavitation with fuel sloshing around with maneuvering...etc etc.

 

I don't want people to think that if you have boost pumps off your engine is going to die after 5-10 minutes.

 

Of course it's all related to how well primed the fuel feed line is and you're not sucking fumes or the last 50 lbs of fuel sloshing around the tank. That's what the little DC pump is for. Once the engine is operating and the fuel in the tank is relatively stable both engines will happily keep on burning. And yes this is even on engines installations mounted above the tank like the TF34 or CF34 on CRJs etc.

 

I just don't want developers to think that once we lose the AC Boost Pumps or AC power the engines will shut down in a few minutes. No aircraft would be designed like that - especially one known for it's resilience to battle damage. The engine has to sustain itself for at least a couple hours has been key requirement in aircraft design for a very long time.

Just want to clarify here - the TF34/older CF34 has no problem suction feeding from the fuel tanks. It'll happily run fine at all power settings. There is an operating limitation though of 10 - 50 hours depending on airframe installation. This is because it is hard on the engine driven fuel pump and there is risk of cavitation with fuel sloshing around with maneuvering...etc etc.

 

I don't want people to think that if you have boost pumps off your engine is going to die after 5-10 minutes.

 

Of course it's all related to how well primed the fuel feed line is and you're not sucking fumes or the last 50 lbs of fuel sloshing around the tank. That's what the little DC pump is for. Once the engine is operating and the fuel in the tank is relatively stable both engines will happily keep on burning. And yes this is even on engines installations mounted above the tank like the TF34 or CF34 on CRJs etc.

 

I just don't want developers to think that once we lose the AC Boost Pumps or AC power the engines will shut down in a few minutes. No aircraft would be designed like that - especially one known for it's resilience to battle damage. The engine has to sustain itself for at least a couple hours has been key requirement in aircraft design for a very long time.

 

That’s already incorrect in the sim. They will not suction feed at max like they should.

 

I’m going to try the FCF sortie again next weekend but I doubt it’s changes.

 

https://forums.eagle.ru/showthread.php?t=262671

Just want to clarify here - the TF34/older CF34 has no problem suction feeding from the fuel tanks. It'll happily run fine at all power settings. There is an operating limitation though of 10 - 50 hours depending on airframe installation. This is because it is hard on the engine driven fuel pump and there is risk of cavitation with fuel sloshing around with maneuvering...etc etc.

 

I don't want people to think that if you have boost pumps off your engine is going to die after 5-10 minutes.

 

Of course it's all related to how well primed the fuel feed line is and you're not sucking fumes or the last 50 lbs of fuel sloshing around the tank. That's what the little DC pump is for. Once the engine is operating and the fuel in the tank is relatively stable both engines will happily keep on burning. And yes this is even on engines installations mounted above the tank like the TF34 or CF34 on CRJs etc.

 

I just don't want developers to think that once we lose the AC Boost Pumps or AC power the engines will shut down in a few minutes. No aircraft would be designed like that - especially one known for it's resilience to battle damage. The engine has to sustain itself for at least a couple hours has been key requirement in aircraft design for a very long time.

 

I thought I had it right the first time... dang. We did have a shutdown at idle but the more I think about it yet again the valve at the tank (trailer tank) was in shutoff mode and starved the supply. Thanks for the clarification and knowledge reinforcement.